A fuel cell has been proposed as a clean, efficient and environmentally responsible power source for electric vehicles and various other applications. Individual fuel cells can be stacked together in series to form a fuel cell stack. The fuel cell stack is capable of supplying a plurality of electricity sufficient to power a vehicle. In particular, the fuel cell stack has been identified as a potential alternative for the traditional internal-combustion engine used in modern automobiles.
One type of fuel cell is the polymer electrolyte membrane (PEM) fuel cell. The PEM fuel cell includes three basic components: a pair of electrodes, including a cathode and an anode; and an electrolyte membrane. The electrolyte membrane is sandwiched between the electrodes to form a membrane-electrode-assembly (MEA). The MEA is typically disposed between porous diffusion media, such as carbon fiber paper, which facilitates a delivery of reactants such as hydrogen to the anode and oxygen to the cathode. In the electrochemical reaction of the fuel cell, the hydrogen is catalytically oxidized in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The electrons from the anode cannot pass through the electrolyte membrane, and are instead directed to the cathode through an electrical load, such as an electric motor. The protons react with the oxygen and the electrons in the cathode to generate water.
The electrodes of the fuel cell are generally formed from a finely divided catalyst. The catalyst may be any of electro-catalyst which catalytically supports at least one of an oxidation of hydrogen and a reduction of oxygen for the fuel cell electrochemical reaction. The catalyst typically is a precious metal, such as platinum or another platinum-group metal. The catalyst is disposed on a carbon support such as carbon black particles, and is typically dispersed in a proton-conducting polymer, also known as an ionomer. A typical ionomer is a perfluorosulfonic acid (PFSA) polymer, although other ionomer material, including hydrocarbon ionomers such as sulfonated polyetherketones, aryl ketones, and polybenizimidazoles may also be used. One type of perfluorosulfonic acid (PFSA) polymer is commercially available as Nafion® from the E. I. du Pont de Nemours and Company. The electrolyte membrane is likewise formed from an ionomer, typically in the form of a layer.
One known method of forming the electrodes of the fuel cell includes applying a catalyst ink to a suitable fuel cell substrate. An example of a catalyst ink and methods of application is described in U.S. Pat. No. 6,156,449 to Zuber et al., the disclosure of which is hereby incorporated herein by reference in its entirety. The catalyst ink typically contains the catalyst on the carbon support, the ionomer, and a solvent. The catalyst ink is subsequently dried to drive off the solvent and form the electrode. Typical substrates include the electrolyte membrane, such as in a catalyst coated membrane or CCM design, and the diffusion media such as in a catalyst coated diffusion media or CCDM design. Additional known substrates may include polymeric substrates, such as PTFE, ePTFE, and ETFE, for example, which can be laminated to the electrolyte membrane in a decal transfer process.
The typical catalyst ink-fabricated electrode is known to have a network of cracks formed in the surface thereof. The network of cracks is also known as the phenomenon of “mudcracking”. The cracks may form due to non-uniform drying of the catalyst ink during fabrication of the electrodes. The cracks may also form following drying due to an inherent weakness of the electrode. The electrode is formed from a porous matrix of the carbon support bound by the ionomer. The ionomer is a relatively weak binder and the matrix of the carbon support within the ionomer may not be continuous. The carbon support also provides minimal reinforcement to the ionomer, such that the resulting matrix may not withstand substantial stresses during and after the drying of the catalyst ink, resulting in a greater opportunity for the cracks to form during operation of the fuel cell.
The network of cracks can undesirably impact the performance of the fuel cell in a variety of ways. For example, during the typical expansion and contraction of the electrolyte membrane during fuel cell operation, the base of the cracks can form a stress concentration on the adjacent electrolyte membrane which may result in degradation thereof, such as pin-hole formation. The electrolyte membrane immediately adjacent the crack is also exposed to a different humidity environment than the electrolyte membrane immediately adjacent the electrode. The expansion of the electrolyte membrane into the cracks can also degrade the electrolyte membrane, particularly after repeated expansion and contraction cycles. The network of cracks in the electrode can also reduce the effective stiffness of the electrode, resulting in an undesirable movement of the MEA during fuel cell operation.
It is known to minimize the formation of the network of cracks by increasing a time allowed for the catalyst ink to dry. However, increasing the drying time also increases manufacturing costs. Increasing the drying time also does not optimize a resistance to cracking of the electrode during subsequent operation of the fuel cell.
There is a continuing need for a catalyst ink composition for a fuel cell that militates against the formation of a network of cracks in the electrode. Desirably, the catalyst ink composition reduces the occurrence of stress concentrations of the electrolyte membrane, optimizes a durability of the electrode, and provides external reinforcement to the electrolyte membrane.